Step 1.
Select the Rectangular mask tool from the Toolbox clicking on the icon
, or (if Rectangular mask was not the last tool applied) select it from the floating window.

Step 2.
Press the button Normal mode in the tool Options palette.

Step 3.
Bring the cursor to the point of the image where the corner of an imaginary rectangular should be, and press the left mouse button.

Step 4.
Keeping the left button pressed, move the cursor diagonally to the opposite corner and release the button. The selected rectangular area will stay the same, while the unselected area will be filled with a semi-transparent red color.

Ellipse mask

The Ellipse mask tool allows selecting elliptical areas.

Follow the instructions:

Step 1.
Select the Ellipse mask tool from the Toolbox clicking on the icon
, or (if Ellipse mask was not the last tool applied) select it from the floating window.

Step 2.
Press the button Normal mode in the tool Options palette.

Step 3.
Bring the cursor to the point of the image where the corner of an imaginary rectangular with an inscribed ellipse should be, and press the left mouse button.

Step 4.
Keeping the left button pressed, move the cursor diagonally to the opposite corner and release the button. The selected elliptical area will stay the same, while the unselected area will be filled with a semi-transparent red color.

Freehand mask

The Freehand mask tool allows selecting a freehand mask.

Follow the instructions:

Step 1.
Select the Freehand mask tool from the Toolbox clicking on the icon , or (if Freehand mask was not the last tool applied) select it from the floating window.

Step 2.
Press the button Normal mode in the tool Options palette.

Step 3.
Bring the cursor to the border of the object that needs to be outlined and left-click it.

Step 4.
If you want the contour to be made of straight segments you should keep clicking the left mouse button along the border of the object until the whole object is outlined. If you want the contour to be smooth, move the cursor keeping the left button pressed.

Step 5.
Close the contour by a double-click with the left mouse button. The selected area will stay the same, while the unselected area will be filled with a semi-transparent red color.